This invention relates to an extruder head combined with a preforming die which has a rear surface--as viewed in the direction of extrusion--that is urged into a face-to-face engagement with a support face (abutment face) forming part of the extruder head. The extruder head has at least one flow channel which terminates, open-ended, on the support face.
It is known to exchange a preforming die or an extruder final die in an extruder by displacement in its own longitudinal direction laterally into and out of a recess provided for this purpose in the extruder head. Such arrangements are disclosed, for example, in German Patent Document Nos. 3,629,096 and 3,709,248. Due to the substantial length of the preforming dies (up to 1200 mm), even at high feed rates it takes a significant time period until the preforming die to be replaced is pushed out of the region of the extruder head and a new preforming die is inserted. Since during such a replacement period, a significant amount of material would be extruded and lost, for preforming die replacements in prior art structures the extruder must be stopped. This, however, not only results in an immediate stoppage of production, but it also makes a new adjustment of the extrusion process necessary upon startup. Further, because of the non-stationary flow behavior during the starting phase, a considerable amount of the extruded profiled material is waste.